1. Field of the Invention
The present invention relates to a surface treatment of a copper foil and more particularly to a copper foil for printed circuit boards which has very little deterioration against peel strength due to heat, i.e. a heat resistance against high temperature where the copper foil is adhered to a resin of a base material for the printed circuit boards.
2. Prior Art
Manufacturing techniques for printed circuit boards have progressed rapidly year by year and high efficiency and high reliability are required greatly in manufacturing. These requirements are attended severely to a copper foil as one of the materials for the printed circuit boards.
The first stage of manufacturing the printed circuit boards comprises the steps of laminating a copper foil onto a base substrate into which resin is impregnated, and heating and pressing them together by a pressing process to obtain a copper clad laminate. A glass epoxy substrate, which is used normally and frequently as the base substrate, is formed at 160.degree. C..about.170.degree. C. for 1.about.2 hours, but a higher temperature and longer time are required as conditions for glass polyimide substrate or special resins having high durability which are used increasingly nowadays.
Further, according to progression of surface mounting technology, such techniques are progressed in a direction for necessitating a long time in heating the printed circuit boards due to reflow in soldering, and after the boards are completed, the printed circuit board is required to have a heat resistance due to progression of high density in circuits wherein the temperature rises during operation of the apparatus. As an example, UL796 Standard rules that peel strength of a copper foil should be more than 2 pounds/inch (0.357 Kg/cm) after conducting heat-treatment of 177.degree. C. for 10 days as a test of high temperature heat resistance.
Furthermore, including the heat resistance as mentioned above, the copper foil for printed circuit boards should be of course satisfied with a number of requirements including chemical resistance such as acid resistance after adhesion of the copper foil with the resin of the base boards, the resistance to discoloration of the board after etching (the resistance to Brown transfer), and stainlessness (no residue or spot) after etching.
In order to satisfy the above mentioned requirements for the copper foil for printed circuit boards, the following conventional method of surface treatment has been proposed. For example, Japanese patent publication No. 51-35711 discloses a method of coating a surface of a copper foil with a layer consisting of a group of zinc, indium, and brass and Japanese patent publication No. 53-39376 discloses a method of providing a copper layer of electrodeposition comprising two layers and these are further coated with a zinc layer.
An alloy layer comprising zinc as a main component is superior in heat resistance. However, the layer is very weak in chemical resistance with respect to acid, and therefore after lamination of the copper foil with the resin, undercut occurs in etching, particularly in etching by cupric chloride.
Further, in acid washing or soft etching in a process of manufacturing the printed circuit boards, it occurrs such a problem that peel strength of the copper foil against the resin of the base board becomes lower so that the printed circuit boards may be lifted out of the board.
Although a brass layer has characteristics satisfying both of heat and chemical resistances in a narrow range of composition, any method was never established industrially except using a cyanide bath when providing a brass layer. When using such a cyanide bath, there was a serious problem in public pollution in treating the waste liquid of cyanide from the bath and treating a rinsing water for the brass layer.
Further, if the brass layer is provided by a normal electrolysis utilizing high current density, it departs from a range of appropriate plating so that a powdered deposition is produced. As the result, it occurs such a problem that the powdered deposition sometimes becomes stains after etching or causes peel strength and chemical resistance to be lowered.